Optics deserve attention
Jon Titus, Contributing Technical Editor -- Test & Measurement World, 3/1/2005 2:00:00 AM
Lenses often get little respect. According to Stuart Singer, VP of industrial optics at Schneider Optics, nine out of 10 people he talks with have forgotten to think about lens requirements for a new vision system.
Often, they call at the last minute to find a lens. "By then, they have fixed requirements and have boxed themselves in," said Singer. "They didn't consider lens requirements simultaneous with the selection of their camera." For best results, vision-system designers must begin to determine their lens specifications from a project's start.
"An optical engineer at a lens manufacturer can help designers examine their options for a given camera," added Singer. During analysis of specifications, they might discover no appropriate lens exists. That gives them time to rework a system's design or select a different camera.
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| Suppliers offer varieties of lenses, but you can’t choose one at the last minute. Courtesy of Schneider Optics. |
"Sometimes, I have to tell people they chose the wrong camera," said Singer. They reply they can't change the camera, they cannot invest in custom lenses, and they need to start using the machine-vision equipment right away. But without a lens, there is not much they can do.
The move to larger imaging arrays also can cause problems for system designers. Until recently, 1-in. cameras provided CCD arrays with a 16-mm diagonal. (Inch specs arose from old TV technology and don't necessarily relate to array dimensions.) Manufacturers now provide 22-mm megapixel arrays in cameras with a 1-in. format, and lenses must produce a larger image to cover the larger array surface. Because only a few lenses currently exist for cameras that employ 22-mm sensors, engineers who choose a megapixel camera without considering lens needs are automatically restricting the design flexibility for a vision system.
Even when they understand lens requirements, engineers who quickly spend $5000 to buy a camera often balk at paying $600 or $700 for a high-quality lens. They think a $100 lens will do. "Years ago, my mentor told me, 'Don't throw away what you need to collect—the signal,'" explained Singer. "You must accurately reproduce your object to form an image, and you don't want to lose any photons in the lens."
Singer has a valid point. The same engineers will spend tens of thousands of dollars for a high-end oscilloscope and thousands more for active probes. Yet, they want to skimp on the optical "probe" that makes the difference between success and failure in a vision system.
Part of getting engineers to appreciate the lenses in their systems involves educating them in applied optics and lens technologies. A group such as the Automated Imaging Association (www.machinevisiononline.org) offers tutorials and short courses at its Vision Shows. Engineers can learn from experts who explain optics as they apply to machine-vision systems. Perhaps after some training, vision-system designers will give lenses the respect they deserve. jontitus@comcast.net
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